Generally, as an exhaust purification apparatus provided in an exhaust system of an internal-combustion engine such as a diesel engine, a NOx catalyst has been known that is for purifying the NOx (nitrogen oxide) included in exhaust gas. Regarding this NOx catalyst, various types of NOx catalysts have been known. Among them, a selective reduction NOx catalyst has been known for which an addition of reducing agent is used to continuously reduce and remove NOx. As reducing agent, urea has been known. Typically, urea aqueous solution is injected and supplied to the exhaust gas at upstream-side of the catalyst and heat receipt from exhaust gas or catalyst generates ammonia and NOx is reduced on NOx catalyst by ammonia.
In order to use this selective reduction NOx catalyst, the amount of reducing agent to be added to catalyst must be controlled to an appropriate amount. Thus, a method is used by which a NOx sensor is provided at the downstream side of the catalyst and the amount of the reducing agent is controlled depending on the NOx concentration detected by this NOx sensor.
By the way, in the case of an engine mounted in an automobile for example, in order to prevent the automobile from running with deteriorated exhaust gas, on-board detection of an abnormality of catalyst or a sensor is required by regulations including those in the respective countries. Regarding the detection of an abnormality of catalyst, a relatively large number of techniques already exists. However, as described above, regarding the detection of an abnormality of a NOx sensor provided at the downstream side of NOx catalyst, no effective technique is found in the current situation. In particular, with the current more strict regulations on exhaust gas, it has been required to correctly detect not only a failure such as a disconnection but also the rationality of a sensor output regarding the deterioration for example. Thus, a drastic measure is required that can cope with this.
As this abnormality diagnosis method for a NOx sensor, a method may be considered for example by which a plurality of NOx sensors are provided at an identical position to relatively compare these detection values and a method may be considered by which the NOx sensors are detached and are checked by a fixed analyzer. However, the former case requires a high cost and the latter case fails to provide an on-board diagnosis.
Japanese Laid-Open Publication No. 2003-120399 discloses an abnormality detection apparatus for a NOx sensor that is provided at the downstream side of NOx absorbent. A NOx sensor is determined to be abnormal when the NOx concentration of exhaust gas reaching the NOx sensor is forcedly fluctuated and the fluctuation of the output value of the NOx sensor is different from the fluctuation when the sensor is normal.
However, since the exhaust gas reaching the NOx sensor is exhaust gas having passed NOx absorbent, this exhaust gas has a NOx concentration for which NOx is already absorbed by NOx absorbent. Specifically, the output value from the NOx sensor undesirably reflects an influence by the NOx absorbent in front of the sensor, which causes a deteriorated accuracy of the abnormality diagnosis of the NOx sensor.
Regarding other conventional techniques, Japanese Laid-Open Publication No. 2004-270468 for example discloses an apparatus for causing a NOx storage catalyst to forcedly release NOx during which an actual NOx reduction rate of the NOx storage catalyst is calculated and this actual NOx reduction rate is compared with a reference NOx reduction rate set based on the engine operating condition to thereby determine an abnormality of the NOx sensor. Japanese Laid-Open Publication No. 2004-211631 discloses that, when the deterioration of an exhaust sensor provided at the downstream side of an exhaust purification apparatus is detected, the exhaust purification apparatus is bypassed and exhaust gas is guided to an exhaust sensor. This requires to additionally provide a bypass path for bypassing the exhaust purification apparatus. Japanese Laid-Open Publication No. 2006-118505 discloses that, when a NOx storage catalyst does not provide a NOx storage capacity in an internal-combustion engine in which a NOx sensor is provided at the downstream side of the NOx storage catalyst, a signal of the NOx sensor is compared with a standard to the NOx concentration at the upstream side of the NOx storage catalyst and the NOx sensor signal is corrected. This is a technique regarding a storage-type NOx catalyst and cannot be directly applied to a selective reduction NOx catalyst.
The present invention has been made in view of the above situation. It is an objective of the invention to provide an abnormality diagnosis and an abnormality diagnosis method for a NOx sensor by which an abnormality of a NOx sensor provided at the downstream side of selective reduction NOx catalyst can be favorably detected.